The site is still in development, but so far it has checked 100,000 websites, over 18,000 of which are blocked by one Internet service provider (ISP) or more.

Blocking sites often causes a loss of custom and potential advertising revenue which can severely damage an online business. A site selling and servicing Porsches was blocked by O2 earlier in the year, while the political blog, Guido Fawkes' Order Order, was also blocked by TalkTalk.

Editor of the blog Paul Staines said, "We would really appreciate it if TalkTalk would remove us from their block list. The only people who block us are them and the Chinese government."

The increase in blocked sites is partly the result of new measures brought in by Prime Minister David Cameron to protect children from pornographic and other potentially harmful content.

The new regulations, which were announced in July 2013, were implemented to prevent children from "stumbling across hardcore legal pornography," but there are now concerns that these filters incorrectly block sites that do not contain offensive content.

Executive director of Open Rights Group, Jim Killock, said that the project's aim is to analyse the impact of web filters.

"Already, our reports are showing that almost one in five websites tested are blocked, and that the problem of overblocking seems much bigger than we thought," he said. "Different ISPs are blocking different sites and the result is that many people, from businesses to bloggers, are being affected because people can't access their websites."